Aquatint: A process for enhancing the tonal range of
intaglio plates. A random etched pattern was produced on the
plates by applying resin particles to the plates before
etching. It was called a "ground", and was used as early as
Asphaltum: Synonyms bitumen, pitch, tar. Used by J.N.
Niepce in 1826 for the oldest surviving photograph, and as an
etch resist in various photolithographic processes. It was
usually obtained from Trinidad or the Dead Sea (hence "Bitumen
of Judea"). Pieces broken at temperatures below the softening
range exhibit conchoidal or brittle fracture patterns, unlike
tar from most other sources such as petroleum.
Baryta: Barium sulfate, a natural or synthesized
mineral used as a white pigment; in photography, used as a
paper coating under emulsions to hide paper texture.
Base: This is the bottom supporting material for
photographs. It is one of the attributes listed in Section 1.
The light-sensitive material may be coated directly on the
base, as in salt prints; it may be in an emulsion layer on the
base, or there may be a baryta layer between the base and
Bichromate: The modern spelling is dichromate. The
sensitizer for gum or gelatin processes such as carbon, carbro,
collotype. Sodium, potassium, or ammonium dichromate have been
used, for example K2Cr2O7.
Catalysis: Acceleration of the rate of a chemical
reaction by a substance that does not become a constituent of
the final reaction products. At one time it was thought to
explain the appearance of the visible image in a printing-out
paper, hence the name "catalysotype" in 1844.
Collodion: A solution of gun-cotton in ether and
alcohol; gun- cotton is cotton reacted with nitric acid. It is
highly flammable in liquid form. Towler  has a complete
description. See "Guncotton" below.
Colloid: A suspension of particles in a liquid medium
that fails to settle out. Examples are gelatin and gum arabic.
Colloidal particles are of the order of 1000 times the size of
the molecules of the supporting medium, making them visible
under light microscopy.
DOP, or Developing-Out-Paper: Photographic paper on which
the visible image is chemically developed from an invisible
latent image; first used about 1873, now the predominant type.
Embedded image: The light-sensitive material is soaked
into the paper rather than carried in a binder such as gelatin,
collodion, or albumen. Paper fibers are easily visible in all
parts of the matte image. Examples are salt prints and
Gelatin: Animal derivative first successfully used by
Maddox in 1871 as a binder for silver bromide. It was also used
as a safety film base in stripping films. The old spelling was
Grain: Visible development centers in a photographic
image, not to be confused with paper fiber texture in salt
prints, or the screen pattern in halftone engravings.
Ground: Roughening applied to intaglio plates to aid in
retention of ink. The aquatint process was an example.
Gum Arabic or simply gum: A colloid produced from the
bark of certain trees, used in the gum bichromate process.
Guncotton: The product of the reaction between certain
organic substances such as cotton, and acids such as nitric or
sulfuric. Guncotton is highly inflammable or explosive, and is
soluble in ether and alcohol, yielding collodion, which has
played an important role in photography as an emulsion base.
Eder [48, 342-347] has a detailed discussion of the
Halftone: The complete tonal range from white to black.
A term often applied to inked prints made by various screen
Halide: Chemical compounds containing the halogens
fluorine, chlorine, bromine, and iodine. Silver halides have
been the most important photographic compounds since 1839.
Index of Refraction: A measure of the bending of light as
it passes from one transparent medium to another, where the
velocity of light differs.
Intaglio: ink printing process in which the ink is held
in en- graved recesses below the main surface of the printing
plate, as contrasted to relief printing where raised surfaces
are inked, such as rubber stamps.
Lithograph: a paper print made by oil based inks
transferred from an engraved master on stone.
Latent Image: the invisible chemical change produced by
light in a photosensitive material.
Matte: a surface from which reflected light is
scattered in all directions; rougher than a glossy or smooth
surface. Salt prints and platinotypes have matte surfaces
because the paper is un- coated. Matte or semi-matte surfaces
were produced on coated papers by the addition of starch, or by
mechanical embossing or roughening.
Orthochromatic: A photosensitive surface sensitive to
all colors of the visible spectrum except red; sometimes called
Panchromatic: A photosensitive surface sensitive to all
colors of the visible spectrum.
pH: a measure of acidity or alkalinity of a water
solution, on a logarithmic scale of 0 to 14. Neutrality is 7.0
on this scale; above 7.0 represents alkaline solutions, while
below 7.0 are acids. Alkaline solutions etch most glasses.
"Buffered" paper contains alkaline materials such as calcium
carbonate to neutralize acids that deteriorate paper. The pH
scale is based on hydrogen ion concentration, and is meaningful
only in water solutions; the pH of dry paper must be measured
by certain archival procedures.
Plasticizer: an oil-like chemical added to polymers
("plastics") to make them soft or flexible.
POP or Printing-Out-Paper: photographic paper on which an
image appears spontaneously after light exposure without
chemical development. Excess silver nitrate in the older
emulsions often caused such photolytic development. Examples
are albumen paper and some silver chloride and bromide papers;
still used as proof paper for portraiture.
PPM: Parts Per Million, a measure of concentration,
either by weight or by volume. Example: 0.1% = 1000 PPM.
Provenance: documentation on the known history of an
Resin: (1) Natural organic solids secreted from plants;
example - rosin from pine trees. (2) Synthetic organic polymers
used as "plastics".
Reticulation: a microscopic worm-like pattern in
gelatin emulsions resulting from rapid and extreme temperature
changes in solution, or drastic acid-alkaline cycling. It is a
damage condition that is sometimes used for special effects. It
was deliberately used in the collotype process to produce a
random screen for halftone printing.
Sizing: a treatment applied to paper to produce a
smooth base for subsequent coatings, to improve wet strength,
and to reduce absorption of chemicals into the paper fibers.
Many materials have been used, such as animal glue, tapioca,
arrowroot, and gelatin, as well as modern resins.
Specular: Reflection of a coherent image from a smooth
surface such as a mirror, as opposed to diffuse light from a
matte surface. The direction of reflection is determined by the
direction of the incident light, which can only occur when the
height of irregularities does not exceed a small fraction of
the wavelength of light.
Thermoplastic: a polymer whose solid shape can be
reversibly altered by the action of heat and pressure. Examples
are poly- vinyl chloride ("vinyl"), and polymethyl methacrylate
Thermosetting plastic: a polymer whose shape cannot be
altered by the action of heat and pressure without the
occurrence of decomposition. Examples are epoxies, and phenolic
resins such as Bakelite.
Translucent: An optical property that passes diffuse
light but not clear images.